Encode a URL to a percent-encoded form, excluding already-encoded sequences.

This function will take an already-encoded URL and encode all the non-URL code points. This function will not encode the "%" character unless it is not part of a valid sequence (%20 will be left as-is, but %foo will be encoded as %25foo).

This encode is meant to be "safe" and does not throw errors. It will try as hard as it can to properly encode the given URL, including replacing any raw, unpaired surrogate pairs with the Unicode replacement character prior to encoding.